How To Reform Electrolytic Capacitors

Many aluminum electrolytic capacitors becomedefective because they sit unused for extendedperiods of time. This Tech Tip will explain whythis phenomenon occurs and how you can re-form these electrolytics with your Z Meter.

What Is An Aluminum ElectrolyticCapacitor?

Aluminum electrolytic capacitors are among themost common capacitors used today. These ca-pacitors have become popular because theyoffer a large capacitance in a small package.This makes them ideal to use as filters in powersupplies and couplers in low frequency circuits.However, aluminum electrolytic capacitors arealso the least reliable capacitors on the market,because eventually every one of them will failbecause of their construction, which is explained

1later.

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An aluminum electrolytic capacitor consists oftwo aluminum foil plates separated by a porousstrip of paper soaked with a conductive solutioncalled the “electrolyte”. This moist paper, how-ever, is not the capacitor’s dielectric. It serves asa spacer to prevent the two plates from mechani-cally shorting. The electrolyte conducts the chargefrom the negative plate, through the paper, andinto direct contact with the dielectric.

The dielectric is a thin layer of aluminum oxidethat is electrochemically formed on the positivefoil plate. The key to an electrolytic is the fact thatthe capacitance increases linearly as the thick-ness of the dielectric decreases. Therefore,because of their extremely thin dielectric, elec-trolytics can have a large capacitance within asmall package. Figure 1 shows an electrolytic’sconstruction.

After the capacitor is formed, it is rolled up andplaced in a cylindrical case. The case is then

stamped with a manufacturing label. Most ca-pacitors have the manufacturing date printed onthe case along with the label. This date is printedin the form of a four digit number. The first twodigits represent the year that the capacitor wasproduced, and the last two numbers signify theweek of the year it was produced. For example acapacitor with a date code of 8318 stamped onits case was manufactured in 1983 during the18th week. By checking this date you can deter-mine exactly how long a capacitor has beensitting on a shelf.

Fig. 2: The date code, found on most electro-lytics, is a four-digit number indicating theyear and the week the capacitor was made.

What Goes Wrong WithElectrolytics?

Fig. 1: As an aluminum electrolytic ages on the shelf, the aluminum-oxide dieletric materialdissolves into the electrolyte causing leakage and dielectric absorption.

Since the electrolyte inside the aluminum elec-trolytic capacitor is water-based, it will eventuallydry out, causing capacitor malfunction. The

or 39G201 TEST BUTTON HOLD DOWN RODbetween the button and the handle.

4. Adjust the length of the rod by holding one endand turning the other until the hold down rodkeeps the CAPACITOR LEAKAGE button de-pressed. (The 39G145 is preset for you.)

5. Apply the Z Meter’s leakage voltage to the ca-pacitor until the leakage reading is below themaximum acceptable leakage level for the valueand voltage rating of the capacitor you are test-ing. Note: Some electrolytics may take only fiveminutes to reform while some may take an houror more.

6. After the capacitor leakage has droppedbelow its maximum rated value, allow it to set for30 minutes. Then recheck the value and leakageto see if reforming has improved the capacitor. Ifthe capacitor checks bad once again, considerthe capacitor bad and discard it. If it checks good,put it in circuit for normal use.

If the capacitor leakage does not drop to an ac-ceptable level after several hours of reforming,

Fig. 3: Use the Z Meter's leakage voltage to reform electrolytics that have aged because of non- remove the Z Meter and throw the capacitoruse.

capacitor is good, or at least reformable, as longas the electrolyte remains liquid.

When the electrolyte drys out, however, or thealuminum oxide dielectric layer deteriorates, theleakage goes up and the capacitor loses capaci-tance. This can happen to aluminum electrolyt-ics just sitting on the shelf, in fact it’s more likelyto happen to an electrolytic that sits idle.

When an aluminum electrolytic starts this dete-rioration process, the first symptoms to look forwith your Z Meter are larger amounts of dielectricabsorption and leakage, although ESR may bepresent, too. These symptoms do not occur,however, at a linear rate over time. Once the ca-pacitor starts to show these symptoms, thecapacitor only has a short life left unless it isreformed quickly.

Reforming Aluminum Electrolytics

In many cases, the capacitor’s electrolyte solu-tion has not dried up. Instead, the dielectric oxidecoating has deteriorated with time, causing prob-lems in the capacitor. The oxide can be re-formed by applying a voltage to the capacitor’splates. By using your Z Meter’s leakage power

supply test voltage on the capacitor, the electro-lyte experiences a chemical reaction that helpsrestore the dielectric oxide to its original state.

Use the supplied TEST BUTTON HOLD DOWNROD and your Z Meter to apply the rated leakagevoltage to the capacitor for extended periods oftime. The hold-down rod fits between the CA-PACITOR LEAKAGE test button and the carry-ing handle to hold the LEAKAGE button in. Oncethe leakage current of the capacitor stays belowthe maximum allowable leakage limit, you mayuse the capacitor in-circuit. If the reforming proc-ess does not reduce the leakage current to anacceptable level, the electrolytic is defective andshould be discarded.

To reform an aluminum electrolyticcapacitor:

1. Connect the capacitor to the Z Meter testleads.

away.

2. Choose or enter the Z Meter leakage voltage(the capacitor’s working voltage or next lowestvoltage step).

3. Depress the CAPACITOR LEAKAGE button,and while holding it in, place the 39G145, 39G154, Form 4341 Printed In U.S.A.